On Windows Server 2012 R2, the microsoft-windows-netfx3-ondemand-package.cab file is the primary component for installing .NET Framework 3.5. Because this feature is "removed" by default to save disk space, you must provide an external source (like installation media) or configure the server to download it from Windows Update. Installation Methods 1. Using DISM (Command Line)
Third-Party Alternatives: Some community forums provide the standalone CAB file, which is approximately 69 MB, but using official Microsoft media is recommended for security and compatibility. Step-by-Step Installation Guide Mount the Server 2012 R2 ISO
/LimitAccess: Prevents the server from trying to contact Windows Update, which often causes the process to hang. Method 2: Server Manager (GUI) Open Server Manager and click Add Roles and Features. On Windows Server 2012 R2
Install-WindowsFeature -Name NET-Framework-Core -Source D:\sources\sxs